17... a8d8  18. a3c2 Both sides play natural moves. The knight returns to a better position. 18... d7f8 Preparing to attack the d5-pawn and maybe play Nf8-e6, exploiting the pin. There are alternatives. 19. b2b4 An interesting decision. By breaking the queenside, White gains important squares for the knight. 19... c5c4 White gets the d4-square for the knight, but the passed pawn on c4 gives Black reasonable chances.